Device for dynamic measurement of the surface tension of a liquid

ABSTRACT

The invention relates to a device for dynamic measurement of the surface tension of a liquid by the bubble pressure process. The device is characterized by a moving measuring device with a flexible lead, conveying at least compressed air, to a sensor head which releases bubbles and can be immersed in the measuring liquid. The moving measuring device has an input keyboard for different operating modes, a display for monitoring the measuring modes and display of the measurement results, a volume flow source for generating the gas pressure, a pressure sensor for detecting the quality of gaseous bubbles, a microprocessor for measuring and evaluating the measurement results and an internal current supply for all power consumers.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a device for dynamic measurement of thesurface tension at boundary surfaces between liquids and gassesaccording to the method of evaluation of the maximum bubble pressure.

2. Description of the Related Art

A device of this type and a measurement process for this is described ingreater detail in, for example, EP 0 149 500 A2.

For the theory and for further understanding of the measurement method,reference is made to the company publication: BlasendrucktensiometerBP2, Benutzerhandbuch, Krüss GmbH, Hamburg 1995.

The mentioned devices are suitable for stationary laboratory testing ofliquids. Very high demands are made on the measurement conditions. Thus,for example, the state of fullness or, rather, the immersion depth ofthe capillary in the measurement liquid must be adjusted with millimeterprecision with the aid of a sinking device, in which process a heightadjustment up to the “jumping” of the liquid onto the capillary mustfirst be undertaken manually and with very great care. The installationlocation is to be chosen with great care, since, for example, vibrationsdue to personnel walking by or drafts ensure a serious adulteration ofthe measurement results. Also, the measurement apparatus is to becarefully aligned. By reason of the demanding design of the measurementplace and the high weight of about 20 kg, mobile application isprohibited. A further reason for the fixed design of the measurementplace is that for the generation of bubbles relatively high pressuresare required, for which, in turn, large external pressure-gas producersmust be available and, obviously, an electrical connection must bepresent. The regulation of the bubble formation and/or bubble frequencytakes place in a demanding manner by means of analog valves. Theevaluation can take place only on a personal computer.

Furthermore, devices are known that for the purpose of continuousmeasurements are in constant contact with a particular liquid to betested (DE 41 12 417 A1, DE 43 03 133 A1), and which are very expensive.An external air compressor, two compressed-air hoses, two valves, andtwo different precision capillaries, immersed to exactly the same depth,are necessary for the generation of the gas bubbles, as well as apressure-difference gauge at the capillary feeds. The evaluation takesplace on a personal computer. Disturbances, caused by a bubble break-offat one of the capillaries, makes an evaluation difficult.

In all known devices the pressure or rather the pressure differencebetween two capillaries is measured as an absolute value, for whichpurpose relatively cost-intensive pressure sensors with a very precisecalibration are required.

Finally, from DE 44 23 720 C1 is known a generically different devicefor measuring the surface tension of preferably molten metals, with onecapillary for the gas supply, which capillary is conducted for examplevertically through the bottom of a crucible receiving the molten metaland terminates at a nozzle for the formation of the gas bubbles. Withknowledge of the surface tension of cast iron, by means of this deviceconclusions can be made regarding the graphite morphology of the carboncontained in the cast iron, and the sulfur content of the pig iron oreven the refinement processing of aluminum-silicon alloys can be judged.

It is a matter here of an elaborate apparatus to be operated in astationary manner, with which apparatus the frequency of the gas bubblesemerging from the nozzle into the molten cast iron is determined. Therelatively long capillary with an inner diameter of only 0.7 to 1.5 mmcauses a considerable flow resistance for the gas the thus necessitatesa high expenditure of energy during the measuring process. Beyond that,during the operation the moistening characteristics and the innerdiameter of the capillary are altered by the measurement meltpenetrating more or less into the capillary, and thus the measurementparameters are also altered uncontrollably, which leads, finally, touncertain measurement results. Moreover, after every measurement acareful purging of the capillary or, better, its replacement isnecessary. Both of these are time-consuming and expensive.

SUMMARY OF THE INVENTION

The task of the invention is to create a compact device for a nearlyuniversal, simple, reliable and low-maintenance use. The device shouldmake possible good measurement precision even with relatively largefill-state tolerance of the liquid in the container, and have a lowpower usage, a low weight, and greatly reduced production costs. Inparticular, the carrying out of its application should be possible in amobile manner, whereby the constant sending in of test samples to alaboratory is just as dispensable as with an apparatus that is bound toone installation. Further, it should be possible to measure even thesmallest amounts of liquid.

The task is accomplished through the characterizing features of thefirst claim. Advantageous further developments are specified in thedependent claims.

With the apparatus according to the invention, industry is given asmall, light measurement device that is not dependent on an electricalmeans or air pressure, in short is mobile, that is relatively robust andsimple to operate, and nevertheless is capable of delivering goodmeasurement results with low acquisition cost and universal application.The measurement apparatus is operated via an input keypad and hasdifferent operating modes. The selected operating mode and themeasurement results and/or error messages can be read off from adisplay. An internal volumetric-flow source generates the necessary gaspressure, the quality of the gas bubbles is measured by an internalpressure sensor, and the evaluation takes place by means of amicroprocessor. An internal power supply supplies all power consumersindependently of an electrical means as desired. It consists of a110/220-volt power unit and/or a rechargeable or expendable battery.

Used advantageously as a volumetric-flow source is a controllablelow-voltage membrane pump that is capable of building up a sufficientlyconstant gas pressure. When the nozzle according to the invention isused, the necessary gas pressure amounts to only about {fraction (1/10)}of the pressure in the standard devices.

For the pressure sensor is used, according to an especially preferredimplementation, a sound-pressure transducer, namely a mini-microphone.This is cost-effective and delivers at its output the first derivativeof the measured bubble pressure, thus a measurement signal that isindependent of the immersion depth of the nozzle.

The measurement apparatus has four selectable operating modes, namely acalibration mode, a first measurement mode for surface-tensionmeasurement with a constant, selected bubble frequency, a secondmeasurement mode for surface-tension measurement with automaticbubble-frequency through-flow, and a purging mode, as well as anadditional error mode.

According to an advantageous implementation, a nozzle directed towardthe surface of the liquid is installed in the sensor head, the length ofwhich nozzle is very short in relation to the nozzle opening. By thismeans an undisturbed bubble break-off is produced in the direction ofthe lifting force of the bubbles, which contributes to the precision ofmeasurement. Moreover, the nozzle shape makes a purging considerablyeasier, and the required gas pressure for the production of gas bubblesin the measurement liquid sinks by an order of magnitude, which is ofparticular significance for a battery-operated, handheld measurementapparatus.

Obviously, instead of the measurement nozzle a capillary can be arrangedin the sensor head (1) for generating the gas bubbles.

Since the surface tension becomes lower with a rising temperature, in afurther configuration of the invention provision is additionally made inthe sensor head for a temperature sensor for the evaluation of themeasurement results.

In addition, in the region of the measurement liquid a conductivitysensor can also be arranged in the sensor head, in order to measure theconductivity of the measurement liquid simultaneously with themeasurement of the surface tension.

A fill-state sensor in the sensor head is indispensable if a staticpressure sensor is employed.

For especially exact measurements, it is of advantage to fasten thesensor head (1) into a holder.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects and features of the present invention will become apparentfrom the following detailed description considered in connection withthe accompanying drawings. It is to be understood, however, that thedrawings are designed as an illustration only and not as a definition ofthe limits of the invention.

In the drawings, wherein similar reference characters denote similarelements throughout the views:

FIG. 1 shows: the functional structure of a handheld measurementapparatus

FIG. 2 shows: a structural implementation of the collective apparatus

FIG. 3 shows: the structure of a sensor head according to the invention

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A sensor head 1 is connected to a handheld measurement apparatus 3 via aflexible supply duct 2. In the sensor head 1 (still to be described ingreater detail), into which a measurement liquid 4 can flow upon theimmersion of the sensor head 1 into measurement vessel 5, is situated anozzle 6, through which the bubbles can be forced into the measurementliquid 4 via the supply duct 2. The supply duct 2 consists of a flexiblehose 7 end, if necessary, of current-conducting, insulated strandedwires 8 for supplying additional sensors 9, 10, 19, 20, 21.

In the housing of the handheld measurement apparatus 3 is accommodatedas a volumetric-flow source a uniformly-operating, controllable membranepump with a power consumption of fewer than 5 watts. The membrane pumpis connected to a power supply 12, a 110/220-volt power unit and/or astorage cell, or rather, a battery. Via a T-piece in the hose 7 the airpressure built up by the membrane pump reaches a pressure sensor 13,advantageously a sound-pressure transducer, as well as a nozzle 6, wherea bubble develops. The sound-pressure transducer converts the staticpressure portion of the bubble pressure into a measurement signaldifferentiated according to time, whereby the measurement becomesindependent of the fill state of the measurement vessel, and/orindependent of the depth of immersion of the sensor head 1 in themeasurement solution 4, which is an essential prerequisite for ahandheld measurement apparatus 3 that is to be operated in anuncomplicated manner. Also, a sound-pressure transducer is significantlymore cost-effective than a conventionally-applied pressure sensor thatmust be calibrated in an elaborate manner. Through integration over timein a microprocessor 14, the bubble pressure and thus the surface tensionof the liquid 4 can be easily determined. Moreover, the measurementmagnitudes, bubble frequency, temperature and, if necessary, theconductivity of the measurement liquid 4 can be displayed on an LCDdisplay 15 on the handheld measurement apparatus 3 and stored in themicroprocessor 14, along with the data and time of the measurement. Viaa computer interface, e.g. RS 232, a transmission to an externalpersonal computer of all measurement values for representation andfurther processing is possible as an option. The operating of thehandheld measurement apparatus 3 takes place by means of an input keypad16.

The sensor head 1 has a sensor housing 17, in whose lower region thenozzle 6 is arranged. The nozzle 6 has a very long length in relation tothe nozzle opening, whereby the disturbing effect of capillary forcesdependent of boundary-surface tension, as occur with measurementcapillaries, remains essentially without influence. Also, through thismeans a purging of the nozzle 6 is made considerably easier and thenecessary gas pressure significantly lessened.

The nozzle 6 is directed towards the surface of the measurement liquid4, so that the break-off direction corresponds to the natural directionof buoyancy of the gas bubble, whereby measurement errors are reduced.Serving the gas feed is a channel 18 passing through the sensor housing17, which channel has a mouth underneath the nozzle 6.

Further, in the region of the moistening by the measurement liquid 4 atemperature sensor 9 is arranged and connected to the microprocessor 14in the handheld measurement apparatus 3 via a stranded wire 8 guidedthrough the supply duct 2. The registering of the temperature and, ifneed be, a temperature compensation is important for reasons of theessentially linear dependence of the surface tension on temperature,because with the handheld measurement apparatus 3 measurement liquids 4between 10° C. and 90° C. are to be diagnosed.

On the bottom of the sensor housing 17, still lower than the mouth ofthe channel 18, is advantageously arranged a ceramic moisture orconductivity sensor 19. In the space between this sensor 19 and thenozzle opening there is normally no measurement liquid 4. The sensor 19monitors a possible penetration of measurement liquid 4 into the nozzle6 and if this is the case gives a signal to the microprocessor 14.

Furthermore, above the nozzle 6 can be situated a conductivity sensor10, preferably capacitive, whose electrodes are attached to the insidewall of the sensor housing 17. Applied to the electrodes, in a knownmanner, is an alternating voltage. The capacitor consisting of theelectrodes and the measurement liquid 4 located between them produces anelectrical impedance, from which impedance measurement values regardingthe concentration of, for example, cleansing agents in the measurementliquid 4 can be ascertained. With this, the possibility of determiningat the same time both the surface tension and the conductivity of themeasurement liquid 4 is created.

Finally, fill-state sensors 20, 21 can be accommodated in the sensorhousing 17, which sensors signal the level of the measurement liquid 4in the sensor head 1. Although the measurement is largely independent ofthe liquid level, naturally sufficient liquid 4 must be present todevelop a homogeneous bubble stream.

First Measurement Mode

The handheld measurement apparatus has four operating modes and oneerror mode, which are selectable via the keypad 16.

1. Calibration Mode

In the calibration mode, a system check takes place first and then thecalibration of the handheld measurement apparatus 3 to the surfacetension of a know liquid, e.g. water or alcohol.

2.

In measurement mode 1 the surface-tension measurement takes place withthe bubbles emerging at a constant frequency; preferably, bubblefrequencies of 1 Hz to 10 Hz are selectable. However, wider frequencyranges are also possible.

3. Second Measurement Mode

In second measurement mode the surface-tension measurement takes placewith an automatic frequency through-flow of the bubble emergence from 1Hz to 10 Hz. Wider frequency ranges are also possible.

4. Purging Mode

A purging of the nozzle 6 takes place with the full volumetric-flowpower of the volumetric-flow source 11.

5. Error Mode

The handheld measurement apparatus 3 automatically recognizes errorsoccurring in operation and handling, as well as during measurement, sothat erroneous measurements are excluded to the greatest degree. Theerrors are indicated on the LCD display 15. Thus, measurement liquid 4penetrating the nozzle 6 is registered and indicated. Furthermore, theturning off of the handheld measurement apparatus 3 by means of an OFFkey is only possible if the sensor head 1 has been removed from themeasurement liquid 4. By this means, for all practical purposes themeasurement liquid is prevented from getting into the nozzle 6. Further,the exceeding of or falling below the measurement region of the surfacetension, which region preferably lies between 15 and 80 mN/m, thetemperature of the measurement liquid 4 between 0° C. and 100° C., andthe ambient temperature are indicated.

Accordingly, while only one embodiment of the present invention has beenshown and described, it is obvious that many changes and modificationsmay be made thereunto without departing from the spirit and scope of theinvention.

One field of application is, for example, the diagnosis ofsurfactant-containing suds or rinse waters, another is the qualityassurance of surfactant-containing inks, pigments, cleansing media, orwiper fluids. Among other things, with the device according to theinvention a very good diagnosis of pesticide, photochemical, andpharmaceutical solutions is possible. Additional fields of applicationopen up in the semiconductor, metal-processing, and textile industries.

What is claimed is:
 1. A mobile device for dynamically measuring thesurface tension of a liquid using a bubble pressure method comprising: ahand held measuring instrument comprising: a micro processor forcontrolling and processing measurements; a display screen connected tosaid microprocessor for displaying measured results; an input keyboardconnected to said display screen for selecting modes of measurement,calibration and cleaning of the surface tension; a volume stream sourceconnected to said microprocessor for producing the bubbles; a pressuresensor connected to said microprocessor for detecting measurementsignals; and a power supply connected to said microprocessor; and asensor head having a nozzle for releasing bubbles into the liquid, saidsensor head bring connected to said measuring instrument by a flexiblefeed line that feeds pressurized gas from said volume stream source tosaid sensor head.
 2. The device according to claim 1, wherein saidmicroprocessor is equipped with a first measurement mode for measuringthe surface tension with constant, selectable bubble frequency and asecond measurement mode for measuring the surface tension with anautomatic bubble frequency cycle.
 3. The device according to claim 1,wherein said pressure sensor is designed as a sound pressure converter.4. The device according to claim 1, further comprising a filling levelsensor in said sensor head.
 5. The device according to claim 1, whereinsaid sensor head is secured with a holding device.